The present invention relates to a method for fabricating a semiconductor nanocrystal which is used as a floating gate to be used for electrically erasable and programmable nonvolatile memories and the like, and also relates to a semiconductor memory device using the semiconductor nanocrystal.
For reduction in power consumption as well as reduction in size of electronic equipment, it is considered that a semiconductor memory device (EEPROM) which is high in degree of integration, low in power consumption and electrically erasable and programmable is necessary. This semiconductor memory device having nonvolatility has a floating gate between a channel region and a gate region, this floating gate serving as a carrier confinement region. However, the semiconductor memory device generally has the following problems:
(i) In terms of reliability deterioration due to hot carriers, the number of times that electric charges are implanted to and removed from the floating gate is limited, so that the number of writing and erasing operations is limited; PA0 (ii) A relatively thick insulating film is needed to maintain the nonvolatility, and in order to implant electrons or positive holes through this thick insulating film by Fowler-Nordheim tunnel effect, a voltage as large as 10 V or more is demanded as it stands. As a result, hot carriers are generated so that degradation of the insulating film occurs due to the formation of traps by hot carriers, reaction at the interface and the relaxation of hot carriers; and PA0 (iii) Programming and erasing operations are performed with a small current that flows through charging to and discharging from the floating gate, so that the charging and discharging time is long (on the order of milliseconds). PA0 (1) A silicon nanocrystals based memory, Sandip Tiwari et al., Appl. Phys. Lett. 68(10), p.1377 (1996) PA0 (2) Fast and Long Retention-Time Nano-Crystal Memory, Hussein I. Hanafi et al., IEEE Trans. Electron Device, Vol. 43, p.1379 (1996)
Under these circumstances, a semiconductor memory device that has solved these (i) to (iii) problems has been proposed in Japanese Patent Laid-Open Publication HEI 7-302848. In this semiconductor memory device, as shown in FIG. 5, a source region 108 and a drain region 110 are formed with a specified spacing in a semiconductor substrate 120, and a floating gate 104 is formed on the semiconductor substrate 120 via an insulating layer 112 in a region opposite to a channel region 106 between the source and drain regions 108, 110. Then, the floating gate 104 is covered with an insulating layer 102, and a control gate 100 is formed thereon. The floating gate 104, as shown in FIG. 6, is formed into a cluster or island 122 made of a semiconductor material having a diameter of 1 nm-20 nm. Further, the insulating layer 112 between the channel region 106 and the floating gate 104 is thinned until electrons are allowed to pass therethrough directly by the tunnel effect, while the floating gate 104 is made lower in energy level than the channel region 106, thus making it hard for trapped electrons to easily escape therefrom.
The method for fabricating the floating gate is described in the following two literatures:
FIG. 7 schematically shows a cross section of a semiconductor memory device having a floating gate described in this literature. A 1.1 nm-1.8 nm thick tunnel insulating film 202 is formed on a semiconductor substrate 201 having a source region 206 and a drain region 207 formed therein, and nanocrystals 203 with diameter 5 nm and spacing 5 nm are formed on the tunnel insulating film 202 with CVD (Chemical Vapour Deposition) equipment. The density of the nanocrystals 203 is 1.times.10.sup.12 cm.sup.-2. Further, a control gate insulating film 204 is formed on the nanocrystals 203, and 7 nm thick SiO.sub.2 is deposited on the control gate insulating film 204, by which a control gate 205 is formed.
FIGS. 8A to 8C show a method for fabricating a semiconductor memory device having a floating gate as described in this literature. A 5 nm-20 nm thermal oxide film 302 is formed on a semiconductor substrate 301 (shown in FIG. 8A), and high-dose silicon Si or germanium Ge is ion-implanted to an oversaturation into the thermal oxide film 302 (shown in FIG. 8B). The ion implantation in this case is carried out under conditions of, for example, 5 keV and 5.times.10.sup.15 cm.sup.-2. After that, in a nitrogen N.sub.2 atmosphere, heat treatment at 950.degree. C. for 30 minutes is carried out, by which 5 nm-diameter nanocrystals 303 of silicon Si or germanium Ge are grown in the thermal oxide film 302.
Then, a source region 305 and a drain region 306 are formed in the semiconductor substrate 301 with a specified spacing, and a gate electrode 304 is formed on the thermal oxide film 302 at a region opposite to the region between the source region 305 and the drain region 306 (shown in FIG. 8C).
As described in the above literatures (1) and (2), a shift voltage .DELTA.Vth of a threshold voltage Vth which results when one electron is stored per nanocrystal can be expressed by the following equation: EQU .DELTA.Vth=q(n.sub.well /.epsilon..sub.ox) (t.sub.cntl +(.epsilon..sub.ox /.epsilon..sub.si)t.sub.well /2) (Eq. 1)
where
q is the charge of the electron; PA1 n.sub.well is the nanocrystal density; PA1 .epsilon..sub.ox is the dielectric constant of the oxide film; PA1 t.sub.cntl is the thickness of the control gate oxide film; PA1 .epsilon..sub.si is the dielectric constant of silicon; and PA1 t.sub.well is the size of nanocrystal.
As apparent from Equation 1, it can be understood that variations in the device characteristic (.DELTA.Vth) can be reduced by reducing variations in the nanocrystal density n.sub.well and the size of nanocrystals t.sub.well. Also, since the thickness of the tunnel insulating film between the nanocrystals and the channel determinatively conditions the direct tunneling of electrons to the nanocrystals (the tunneling probability is expressed by a function of the thickness of the tunnel insulating film), variations in this film thickness affects variations in the programming characteristics. Thus, the nanocrystal density, the size of nanocrystals and the thickness of the tunnel insulating film between the nanocrystals and the channel can be considered as principal parameters inherent in memory which are to be controlled.
Now the nanocrystal density, the size of nanocrystals and the thickness of the tunnel insulating film between nanocrystals and channel in the aforementioned literatures (1) and (2) are discussed. On Literature (1):
The semiconductor memory device of Literature (1) can be regarded as one that utilizes nanocrystals which happen to be present on the surface of the ground SiO.sub.2 film or which are grown in island-like form around random crystal nuclei generated in the initial stage of CVD process, in which case neither the nanocrystal density nor the size of nanocrystals is controlled. As to the thickness of the tunnel insulating film between nanocrystals and channel, because the semiconductor substrate is thermally oxidized in advance, it can be considered that the film thickness can be controlled by the prior art technique. On Literature (2):
In the semiconductor memory device of Literature (2), silicon Si or germanium Ge is ion-implanted into the thermal oxide film 302 and then heat treated, so that nanocrystals are grown in the thermal oxide film 302. In this case, the implanted ion concentration is distributed depthwise, so that the ion concentration in the thermal oxide film 302 cannot be made uniform. Accordingly, because the heat treatment is carried out with variations in concentration distribution, the depthwise nanocrystal density in the thermal oxide film 302 also has a distribution, where it can be considered difficult to control the nanocrystal density, the size of nanocrystals and the thickness of the tunnel insulating film between nanocrystals and channel. That is, it is difficult to improve the controllability and uniformity of the nanocrystal density, the size of nanocrystals and the thickness of the tunnel insulating film between nanocrystals and channel, which are the issues to be solved.
Further, to achieve an ion implantation into an extremely thin oxide film with film thickness 5 nm-20 nm while preventing the ions from reaching the ground semiconductor substrate, there is a need of performing an implantation of ions of as low energy as possible, which is exemplified by 5 keV for a 20 nm oxide film. Besides, the energy needs to be reduced for thinner thicknesses of the oxide film, in which case it could become hard to control the implantation of such low-energy ions with ordinary performance of ion implantation equipment, hence impractical as a fabrication method.
An object of the present invention is therefore to provide a method for fabricating semiconductor nanocrystals capable of forming semiconductor nanocrystals which are highly controllable and less variable in density and size.
Another object of the present invention is to provide a semiconductor memory device which, with the use of the semiconductor nanocrystals for the semiconductor memory device, allows the thickness of the insulating film between semiconductor nanocrystals and channel region to be easily controlled, involves smaller variations in characteristics such as threshold voltage and writing performance, and which is fast reprogrammable and has nonvolatility.
In order to achieve the above object, the present invention provides a method for fabricating semiconductor nanocrystals comprising:
a step for depositing a noncrystal semiconductor thin film on a semiconductor substrate, or on an insulating film formed on the semiconductor substrate, under a low pressure below atmospheric pressure; and
a step for, after the deposition of the noncrystal semiconductor thin film, heat treating the noncrystal semiconductor thin film at a temperature not lower than a deposition temperature of the noncrystal semiconductor thin film in a vacuum or in an atmosphere of a gas having no oxidizability, by which a plurality of spherical semiconductor nanocrystals with a diameter of 18 nm or less are formed on the semiconductor substrate or on the insulating film so as to be spaced from one another.
According to the method for fabricating semiconductor nanocrystals of the present invention, after the noncrystal semiconductor thin film is deposited on the semiconductor substrate or on an insulating film formed on the semiconductor substrate under the low pressure below atmospheric pressure, and then the deposited noncrystal semiconductor thin film is subjected to heat treatment in a vacuum or in a gas atmosphere having no oxidizability at a temperature higher than the deposition temperature of the noncrystal semiconductor thin film so as to be all changed into crystal grains, by which a plurality of spherical semiconductor nanocrystals are formed. In this process, the deposition thickness "t" of the noncrystal semiconductor thin film, the radius "r.sub.0 " of the nanocrystals and the center distance "s" of adjacent crystal grains are in the relation that EQU s.sup.2 t=(4.pi./3)r.sub.0.sup.3 (Eq. 2)
The center distance "s" between adjacent semiconductor nanocrystals is equivalent to the density of the semiconductor nanocrystals and determined by the thickness of the noncrystal semiconductor thin film and the conditions of the heat treatment. Accordingly, by controlling the center distance "s" of adjacent semiconductor nanocrystals and the deposition thickness "t" by the medium of the quality of the noncrystal semiconductor thin film and the conditions of the heat treatment, the density and size of the semiconductor nanocrystals can be controlled. Also, with the diameter of the semiconductor nanocrystals made to be less than 18 nm, the lowest energy of the spherical semiconductor nanocrystals becomes larger than the energy at room temperature, so that the semiconductor nanocrystals as a carrier confinement region can retain electrons for enough long time at room temperature without being affected by thermal fluctuation. Thus, semiconductor nanocrystals which are good at controllability of density and size, and smaller in characteristic variations can be formed. Besides, with the use of the semiconductor nanocrystals for a semiconductor memory device, a semiconductor memory device which allows the thickness of the insulating film between semiconductor nanocrystals and channel region to be easily controlled, involves smaller variations in characteristics such as threshold voltage and programming performance, and which is fast reprogrammable and has nonvolatility can be realized.
In one embodiment, after the deposition of the noncrystal semiconductor thin film, the semiconductor nanocrystals are formed without exposing the noncrystal semiconductor thin film to the air.
According to the method for fabricating semiconductor nanocrystals of this embodiment, after the noncrystal semiconductor thin film is deposited, semiconductor nanocrystals are formed in the absence of any natural oxide film without exposing the noncrystal semiconductor thin film to the air. Because no natural oxide film that blocks the crystal growth is present in the surface under crystallization, the semiconductor nanocrystals crystallize while the surface shape is changing with ease, thus resulting in a shape close to a sphere, which is the most stable shape.
Also, one embodiment comprises a step for, after the deposition of the noncrystal semiconductor thin film, removing an oxide film from a surface of the noncrystal semiconductor thin film at a temperature not higher than the deposition temperature of the noncrystal semiconductor thin film before forming the semiconductor nanocrystals.
According to the method for fabricating semiconductor nanocrystals of this embodiment, even once the substrate is exposed to the air after the deposition of the noncrystal semiconductor thin film, the natural oxide film at the surface is, for example, removed by sputtering with Ar plasma under a pressure below atmospheric pressure or deoxidized and removed in an atmosphere of silane gas or the like under a high vacuum, and then the heat treatment for forming semiconductor nanocrystals is carried out. Thus, the semiconductor nanocrystals crystallize while the surface shape is changing with ease, so that the semiconductor nanocrystals result in a shape close to a sphere, the most stable shape.
Also, one embodiment further comprises a step for, after the deposition of the noncrystal semiconductor thin film, forming crystal nuclei at the surface of the noncrystal semiconductor thin film at a low pressure below atmospheric pressure before forming the semiconductor nanocrystals.
According to the method for fabricating semiconductor nanocrystals of this embodiment, after the noncrystal semiconductor thin film is deposited on the semiconductor substrate or on the insulating film formed on the semiconductor substrate, the crystal nuclei are formed at the surface of the noncrystal semiconductor thin film, and subsequently the semiconductor nanocrystals are grown with the seeds of the crystal nuclei at the surface of the noncrystal semiconductor thin film by heat treatment under a low pressure below atmospheric pressure. Therefore, the controllability for the size, shape, crystallinity and the like of the semiconductor nanocrystals is enhanced so that variations in those characteristics can be further reduced. In this case, the density of crystal nuclei can be determined depending on the conditions under which the crystal nuclei are formed.
Also, in one embodiment, the semiconductor nanocrystals are made from silicon; and
the step for forming the crystal nuclei is carried out in a 0.01 Torr or lower vacuum by using a gas containing any one of silane gas, disilane gas or trisilane gas as a material gas.
According to the method for fabricating semiconductor nanocrystals of this embodiment, after the noncrystal semiconductor thin film is deposited, the substrate is placed in the reaction chamber and treated, while being heated, with the flow of the gas containing any one of silane gas, disilane gas or trisilane gas under a low pressure of 0.01 Torr or lower. Thus, molecules or reaction seeds of the gas are adsorbed to the surface of the noncrystal semiconductor thin film, by which crystal nuclei optimum for the formation of semiconductor nanocrystals can be easily formed without causing the formation of island-like silicon grains. The density of the crystal nuclei can be determined by the temperature and time at and for which the gas containing any one of silane gas, disilane gas or trisilane gas is kept flowing, hence high controllability.
Also, in one embodiment, the semiconductor nanocrystals are made from germanium; and
the step for forming the crystal nuclei is carried out in a 0.01 Torr or lower vacuum by using a gas containing either one of germanium tetrafluoride or monogermane as a material gas.
According to the method for fabricating semiconductor nanocrystals of this embodiment, after the noncrystal semiconductor thin film is deposited, the substrate is placed in the reaction chamber and treated, while being heated, with the flow of the gas containing either one of germanium tetrafluoride or monogermane under a low pressure of 0.01 Torr or lower. Thus, molecules or reaction seeds of the gas are adsorbed to the surface of the noncrystal semiconductor thin film, by which crystal nuclei optimum for the formation of semiconductor nanocrystals made from germanium can be easily formed without causing the formation of island-like germanium grains. The density of the crystal nuclei can be determined by the temperature and time at and for which the gas containing either one of germanium tetrafluoride or monogermane is kept flowing, hence high controllability.
Also, in one embodiment, the semiconductor nanocrystals are made from silicon and germanium; and
the step for forming the crystal nuclei is carried out in a 0.01 Torr or lower vacuum by using a gas containing any one of silane gas, disilane gas or trisilane gas and either one of germanium tetrafluoride or monogermane as a material gas.
According to the method for fabricating semiconductor nanocrystals of this embodiment, with the flow of a gas containing any one of silane gas, disilane gas or trisilane gas and either one of germanium tetrafluoride or monogermane, treatment is carried out under a low pressure of 0.01 Torr or lower. Thus, molecules or reaction seeds of the gas are adsorbed to the surface of the noncrystal semiconductor thin film, by which crystal nuclei optimum for the formation of semiconductor nanocrystals made from silicon and germanium can be easily formed without causing the formation of island-like silicon-germanium grains. The density of the crystal nuclei can be determined by the temperature and time at and for which the gas containing any one of silane gas, disilane gas or trisilane gas and either one of germanium tetrafluoride or monogermane is kept flowing, hence high controllability.
Also, one embodiment further comprises a step for, after the noncrystal semiconductor thin film is deposited on the semiconductor substrate and the semiconductor nanocrystals are formed, oxidizing the surface of the semiconductor nanocrystals and the surface of the semiconductor substrate, thereby forming an oxide film.
According to the method for fabricating semiconductor nanocrystals of this embodiment, with these semiconductor nanocrystals applied to a semiconductor memory device, after the semiconductor nanocrystals are formed on the semiconductor substrate, the surface of the semiconductor nanocrystals and the surface of the semiconductor substrate are oxidized. Thus, the oxide film which results in a tunnel insulating film between channel region and semiconductor nanocrystals of the semiconductor memory device can be formed with good controllability.
Also, in one embodiment, a deposition thickness "t" of the noncrystal semiconductor thin film and a center distance "s" of adjacent semiconductor nanocrystals satisfy a relationship that t&lt;(.pi./6)s.
According to the method for fabricating semiconductor nanocrystals of this embodiment, the deposition thickness "t" and the center distance "s" of semiconductor nanocrystals are set so as to satisfy the relational expression between the deposition thickness "t" and the center distance "s" of the semiconductor nanocrystals. As a result, semiconductor nanocrystals can be formed with spacings between adjacent semiconductor nanocrystals, without causing the adjacent semiconductor nanocrystals to make contact with each other.
Also, in one embodiment, the semiconductor nanocrystals are made from any one of silicon, germanium or a mixture of silicon and germanium.
According to the method for fabricating semiconductor nanocrystals of this embodiment, semiconductor nanocrystals made from any one of silicon, germanium or a mixture of silicon and germanium can be easily formed by existing fabrication equipment and process control. Moreover, semiconductor nanocrystals which are highly controllable and less variable in the size, shape, crystallinity and the like of the semiconductor nanocrystals can be easily formed.
Also, in one embodiment, the semiconductor nanocrystals are made from silicon;
in the step for forming the noncrystal semiconductor thin film, an amorphous silicon thin film is deposited by using any one of silane gas, disilane gas or trisilane gas as a material gas, or by using a mixed gas of any one of silane gas, disilane gas or trisilane gas and a as having no oxidizability as a material gas; and
in the step for forming the semiconductor nanocrystals, the semiconductor nanocrystals are grown in a 10 Torr or lower vacuum or in a 10 Torr or lower atmosphere of a gas having no oxidizability.
According to the method for fabricating semiconductor nanocrystals of this embodiment, by using any one of silane gas, disilane gas or trisilane gas as the material gas, or by using a mixed gas of any one of silane gas, disilane gas or trisilane gas and a gas having no oxidizability such as helium, nitrogen, argon or hydrogen as the material gas, reaction is made in a vacuum below atmospheric pressure so that a noncrystal semiconductor thin film is deposited, and subsequently heat treatment is carried out at a temperature higher than the deposition temperature of the noncrystal semiconductor thin film in a 10 Torr or lower vacuum or in a 10 Torr or lower atmosphere of a gas having no oxidizability such as helium, nitrogen, argon and hydrogen. Thus, spherical semiconductor nanocrystals uniform in size and shape can be formed.
Also, in one embodiment, the semiconductor nanocrystals are made from germanium;
in the step for forming the noncrystal semiconductor thin film, an amorphous germanium thin film is deposited by using any one of germanium tetrafluoride or monogermane as a material gas, or by using a mixed gas of either one of germanium tetrafluoride or monogermane and a gas having no oxidizability as a material gas; and
in the step for forming the semiconductor nanocrystals, the semiconductor nanocrystals are grown in a 10 Torr or lower vacuum or in a 10 Torr or lower atmosphere of a gas having no oxidizability.
According to the method for fabricating semiconductor nanocrystals of this embodiment, by using any one of germanium tetrafluoride or monogermane as the material gas, or by using a mixed gas of either one of germanium tetrafluoride or monogermane and a gas having no oxidizability such as helium, nitrogen, argon or hydrogen as the material gas, reaction is made in a vacuum below atmospheric pressure, and subsequently heat treatment is carried out at a temperature higher than the deposition temperature of the noncrystal semiconductor thin film in a vacuum with a pressure of 10 Torr or lower or in a 10 Torr or lower atmosphere of a gas having no oxidizability such as helium, nitrogen, argon and hydrogen. Thus, spherical semiconductor nanocrystals uniform in size and shape can be formed.
Also, in one embodiment, the semiconductor nanocrystals are made from silicon and germanium;
in the step for forming the noncrystal semiconductor thin film, an amorphous silicon-germanium thin film is deposited by using any one of silane gas, disilane gas or trisilane gas and either one of germanium tetrafluoride or monogermane as a material gas, or by using a mixed gas of any one of silane gas, disilane gas or trisilane gas, either one of germanium tetrafluoride or monogermane and a gas having no oxidizability as a material gas; and
in the step for forming the semiconductor nanocrystals, the semiconductor nanocrystals are grown in a 10 Torr or lower vacuum or in a 10 Torr or lower atmosphere of a gas having no oxidizability.
According to the method for fabricating semiconductor nanocrystals of this embodiment, by using any one of silane gas, disilane gas or trisilane gas and either one of germanium tetrafluoride or monogermane as the material gas, or by using a mixed gas of any one of silane gas, disilane gas or trisilane gas and either one of germanium tetrafluoride or monogermane and a gas having no oxidizability such as helium, nitrogen, argon or hydrogen as the material gas, reaction is made in a vacuum below atmospheric pressure so that a noncrystal semiconductor thin film is deposited, and subsequently heat treatment is carried out at a temperature higher than the deposition temperature of the noncrystal semiconductor thin film in a vacuum with a pressure of 10 Torr or lower or in a 10 Torr or lower atmosphere of a gas having no oxidizability such as helium, nitrogen, argon and hydrogen. Thus, spherical semiconductor nanocrystals uniform in size and shape can be formed.
Also, one embodiment provides a semiconductor memory device using semiconductor nanocrystals, wherein the semiconductor nanocrystals fabricated by any one of the methods for fabricating semiconductor nanocrystals as defined above are used as a floating gate of a transistor formed on an SOI substrate.
According to the semiconductor memory device using the semiconductor nanocrystals of this embodiment, the semiconductor nanocrystals are used as a floating gate serving as a carrier confinement region of the transistor formed on the SOI substrate. Thus, a fast reprogrammable, nonvolatile semiconductor memory device which can be implemented with smaller numbers of elements and smaller area and which is smaller in variations can be realized.